Boy, your fix'n to open up the proverbial can-o-worms

The debate on hydraulic fluid for our Kubota's is timeless and never ending
Basically, the unwritten law of the higher the cost = the better the quality is just about accurate when it comes to tractor hydraulic fluid. Theres many testimonials w/the vast majority indicating that OEM branded Kubota hydraulic oil is the best, w/exceptions ofcourse.
The "better" exception would be to go w/the more expensive Chevron Syn fluid as those that use it claim better performance and lower whine in their BX's.
At the opposite end of the spectrum theres been quite a few that have went w/the cheaper TSC branded Traveller and JT5 hydraulic oils and went back to OEM and claimed the OEM gave much better performance and quieted the loud HST whine they got w/the cheaper oil.
Then, you've got the inter brand OEM battles

Some will sware to using UDT and only UDT
Others will sware to using SUDT and only SUDT
And now Kubota has released a new Syn hydraulic oil called UDT2.
UDT & SUDT can be mixed per Kubota and not cause any problems. I'm not sure about UDT2 but since it's Syn I would swag that it wouldn't be wise as regular UDT/SUDT would just water it down.
The basic differences on the OEM fluids are:
UDT, straight 20 weight hydraulic oil. Can be used in all Kubota tractors. Best in hot climes.
SUDT, 10w20 multiweight hydraulic oil with better flow @ both lower and higher temps over UDT.
UDT2, Syn hydraulic oil that I "think" is also 10w20 but I haven't seen any specs so I'm not positive. I do know that it is replaceing SUDT and is reputed to cost less than SUDT.
All I've ever run in my 2004 BX23 is SUDT and I've had no problems and do not even notice an HST whine unless I try to pull out w/the brake set or when I go up a hill in high gear.
You would probably be fine if you went w/another brand as long as it's SUDT compliant but, should you encounter an HST problem the dealer "may" try to blame it on the non-Kubota fluid.
I would recommend to ask the dealer which fluid he prepped your BX with (probably SUDT or UDT2) and use that fluid for top off's. I'd also swag that your implement was probably tested at the factory and will have some fluid in it. Could be UDT, SUDT or equivalent so I wouldn't expect you'll see much of a drop in the level when it's connected.
Get a qt and you shoul dbe good to go.
Dave